Etch-back process

ABSTRACT

A shadow mask is initially prepared with apertures dimensioned for printing a latent image of the pattern of apertures onto the faceplate of a color picture tube and, after this has been accomplished, the mask is re-etched to enlarge the apertures to a desired size in a final size relationship from aperture to aperture. An improvement in the re-etch process is achieved by enlarging the apertures of the shadow mask by differentially applying a suitable etchant thereto such that more etchant is applied to a second set of apertures susceptible to enlargement at a slower rate than to a first set of apertures susceptible to enlargement at a faster rate whereby the apertures are enlarged without disrupting the pattern. An apparatus for use with such process is also provided.

1' United States Patent 1191 Lerner ETCH-BACK PROCESS [75] inventor: Martin L. Lerner, River Forest, 111.

[73] Assignee: Zenith Radio Corporation, Chicago,

Ill.

22 Filed: Apr. 14, 1972 211 Appl. No.: 243,945

[52] US. Cl 156/16, 156/11, 156/345 [51] Int. Cl. C23f 1/02 {58} Field of Search 156/8, 11, 16, 345

[56] 7 References Cited UNITED STATES PATENTS 3,502,519 3/1970 White 156/345 X 3,666,579 5/1972 Dietch 156/345 X Primary Examiner-William A. Powell Attorney, Agent, or Firm-Nicholas A. Camasto 1451 Apt-.30, 1974 ABSTRACT A shadow mask is initially prepared with apertures dimensioned for printing a latent image of the pattern of apertures onto the faceplate of a color picture tube and, after this has been accomplished, the mask is reetched to enlarge the apertures to a desired size in ii final size relationship from aperture to aperture. An improvement in the re-etch process is achieved by enlarging the apertures of the shadow mask by differentially applying a suitable etchant thereto such that more etchant is applied to a second set of apertures susceptible to enlargement at a slower rate than to a first set of apertures susceptible to enlargement at a faster rate whereby the apertures are enlarged without disrupting the pattern. An apparatus for use with such process is also provided.

6 Claims, 6 Drawing Figures Etch ant PATENTEMnaomm 3.808071 A A A\ A 55 3 57 56 55 Etc hcmt HOLE SIZE FIG. 5

1 ETCH-BACK PROCESS BACKGROUND OF THE INVENTION The invention is particularly concerned with screening of a color-picture tube characterized by the fact that the viewable phosphor elements are smaller than the apertures of the color-selection electrode. Typical of this kind of tube is that which is known as a blacksurround picture tube. The tube may come in different sizes and configurations, being either rectangular or round. The phosphor pattern usually consists of interleaved deposits of various phosphor materials arranged in the form of dot triads. The present invention however is not particularly affected by the choice of a particular dot triad structure, but for convenience, it will be assumed that the tube in process is rectangular and has dots of red, green and blue phosphor materials arranged to define a multiplicity of dot triads disposed throughout a rectangularly shaped image field.

The tube of the type under consideration differs from the more conventional shadow mask tubes, for example, in that the viewable phosphor deposits are reduced in size. In the conventional tube the phosphor dots are in tangential contact with one another, whereas in the black-surround tube to which this invention is especially directed, the reduced phosphor dots are separated from one another. Their separation provides elemental portions of the screen surface to which a black pigment or other light-absorbing material may be applied, surrounding. each phosphor with the lightabsorbing material and giving rise to its name black surround tube. In its preferred form, the phosphor dots are smaller in diameter than the holes of the shadow mask in order that the cross-sectional area of the electron beams will be larger than the phosphor dots to providea guard band or tolerance for color purity in the picture tube. A tube of this description is the subject of US. Pat. No, 3,164,368 issued Aug. 25, 1964 in the name of Fiore, et al., and assigned to the assignee of the present invention. It has vastly improved brightness and contrast properties.

In the manufacture of tubes of the type under consideration difficulty has been experienced in obtaining the necessary dimensional relation of phosphor dots to electron beam size or cross-sectional diameter. Aneffective solution to the problem which is successful in the commercial production of black-surround tubes has been the re-etch or etch-back process. In accordance with that process the mask has a field of apertures that are etched initially to a predetermined size and in a predetermined size relationship from aperture to aperture for use inphotoresist printing of the screen. This is a well-known printing process which takes advantage of the property of a photosensitive resist which,.upon exposure to actinic energy, changes its solubility in a particular solvent. The most attractive embodiment of the screening process uses a water-based system comprising an aqueous solution of sensitized polyvinyl alcohol which is rendered insoluble in water when exposed to ultraviolet light. After the three phosphor materials have been applied to the screen area in locations precisely determined by exposure through the shadow mask of the color picture tube, that mask is then subjected to a second etching process in order to enlarge the apertures to a size greater than that which is used for screening.

One difficulty experienced in the re-etch or etchback process has been that, although the apertures of the color selection electrode during screening are of the proper size, certain ones of the apertures of the color selection electrode have cross-sections which render them susceptible to enlargement at a faster rate than others. Consequently, uniform processing of the shadow mask during re-etch renders such apertures larger than desired compared to those apertures which enlarge at a slower rate. Therefore, a shadow mask having apertures of proper size in a final desired size relationship from aperture to aperture is difficult if not impossible to obtain using conventional methods.

The present invention is an attractive improvement in that it assures that all of the apertures of the shadow mask are of the proper size in a desired final size relationship from aperture to aperture.

Accordingly, it is an object of the present invention to provide an improved re-etch process which circumvents the prior art difficulties.

It is a particular object of the invention to provide an improved re-etch process which compensates for the differing aperture enlargement rates precipitated by the differing aperture cross-sections, to obtain a color selection electrode which has apertures of the final proper aperture size in a final aperture size relationship from aperture to aperture.

It is an additional object of the invention to provide an improved re-etch apparatus for obtaining a color selection electrode having apertures of the proper size in a final size relationship from aperture to aperture notwithstanding the differing enlargement rates of the apertures in a color selection electrode.

SUMMARY OF THE INVENTION The invention provides in the manufacture of a color cathode-ray tube including an image screen and a shadow mask provided with a multiplicity of apertures arranged in a predetermined pattern, certain ones of the apertures comprising a first set being susceptible to enlargement at a faster rate upon subsequent etching than certain other of the apertures comprising a second set, a method which comprises the steps of printing a latent image of the pattern ofv apertures on the faceplate with the shadow mask and subsequently enlarging the apertures of the shadow mask by differentially applying etchant thereto such that more etchant is applied to the second set than to the first set of apertures whereby the apertures are enlarged without disrupting the pattern.

The present invention also provides an apparatus for re-etching the apertures of a color cathode-ray tube shadow mask of the type-which comprises a multiplicity of apertures arranged in a predetermined pattern, and having an initial predetermined size relationship from aperture to aperture, certain ones of the apertures comprising a first set being susceptible to enlargement at a faster rate than certain other apertures comprising a second set upon exposure to an etchant capable of attacking the color selection electrode. Support means for supporting the shadow mask disposes the shadow mask in a substantially horizontal position. Spray means comprising a plurality of spray nozzles are disposed beneath the shadow mask for differentially applying the etchant thereto, to apply more etchant to the second set than to the first set of apertures to thereby render all the apertures enlarged without disrupting the predetermined pattern.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements and in which:

FIG. 1 is a perspective schematic representation illustrative of a manufacturing apparatus used for manufacturing a shadow mask;

FIG. 2 is a fragmentary cross-sectional view taken on lines 22 of FIG. 1;

FIG. 3a is a fragmentary cross-sectional view to an enlarged scale illustrative of the cross section of one of the apertures produced by the shadow mask manufacturing process;

FIG. 3b is a fragmentary cross-sectional view to an enlarged scale illustrative of the cross section of another aperture produced by the shadow mask manufacturing process;

FIG. 4 is a schematic representation of a shadow mask in position in a re-etch work station embodying the present invention and of the type which may be used to practice the present invention; and

FIG. 5 is a graph which is used in explaining the effects of different aperture cross sections in the re-etch process.

DESCRIPTION OF THE PREFERRED EMBODIMENT A screen of the black-surround tube may be prepared in either of two ways, that is to say, the black-surround pattern may first be developed on the image area followed by the application of interleaved patterns of three different phosphor materials or the phosphors may be applied first after which the areas intervening the phosphor deposits are covered with light-absorbing material. In either event, the shadow mask is used in forming black-surround holes or phosphor deposits that are smaller in diameter than the apertures of the mask as finally installed in the tube. The present invention may be completely understood apart from a discussion of the process for forming the black-surround and therefore, this aspect will not be described.

The technique of applying a multiplicity of phosphor dot triads throughout the image or screen area of a rectangularly shaped color tube is well known to the art and need not be recited in detail here. Briefly, the faceplate section of the envelope, while still physically separated from the conical envelope portion, is chemically cleaned and then coated with a photosensitive material such as a water-base slurry of polyvinyl alcohol sensitized by ammonium dichromate. It is also common practice to include one of the three color phosphors as an ingredient of the slurry. Assume that green phosphor is the first to be applied. The slurry-coated faceplate with shadow mask installed is secured in an exposure position in an exposure chamber or lighthouse. A source of ultraviolet light, positioned to simulate the electron gun of the tube in process that is assigned to excite green phosphor, is energized to direct actinicenergy through the apertures of the shadow mask to the coated image area. The portions of the slurry layer which are exposed are rendered insoluble in water and constitute a latent image of the green phosphor deposit. That image is developed, after the shadow mask has been removed, by washing the screen with water. This leaves deposits of green phosphor in elemental areas distributed throughout the screen or image area. In like fashion, but with slurries that individually have red and blue phosphor ingredients, the red and blue phosphor dots are established. It is, of course, required that the light source, in the application of each of these two additional phosphors, simulate the electron beams of the tube assigned to excite the respective phosphors. This is a well-known photo mechanical printing process in wide commercial use in the production of three-color shadow mask picture tubes. In the printing process the shadow mask serves as a pattern to precisely locate and dimension the various phosphor deposits that constitute the desired phosphor triads. To serve in that capacity, the mask is prepared with a dome shape which approximates a spherical section having a rectangular field of apertures or holes, individually smaller than desired in the final form of the mask but accurately dimensioned for use in screening. To achieve the desired final aperture size, the mask is re-etched after the screening shall have been accomplished and the remainder of this description will confine itself to the reetching steps.

The general principles of chemical milling by which, for example, a metallic workpiece which is etchable in a particular etchant is provided with an aperture pattern are well known. Such a chemical milling apparatus is schematically illustrated in FIG. 1. The metallic workpiece which eventually becomes the shadow mask is in a rolled up configuration such as steel roll 10. It is disposed in a vertical position to be unwound so that the metal sheet may be pulled through the etching apparatus. The apparatus is provided with a plurality of spray nozzles on both sides of the vertically disposed workpiece, spray nozzles 11-15 being on one side and spray nozzles 16-20 on the other. The spray nozzles are disposed horizontally so that the etchant solution, normally ferric chloride, supplied through supply pipes 21 and 22 may be sprayed onto the vertically disposed workpiece in a direction which is normal to the workpiece. Deposited onto both sides of the metal sheet is a coating of material which is resistive to the ferric chloride etchant solution. The resistive material has previously been subjected to an exposure process the result of which can be more clearly seen in FIG. 2. Metal sheet 30 is covered with layers 31 and 32 of etchant resistive material with exposed areas defining the desired aperture dimensions. Spray nozzles and spray the etchant solution on both sides of the sheet. The exposed areas on the side of sheet sprayed by nozzle 15 are substantially larger in diameter than the exposed areas of the sheet on the side being sprayed by spray nozzle 20. This is done to obtain an aperture having a small diameter on one side of the sheet and a larger diameter on the other.

Cross-sectional representations of the apertures produced by this process are shown in FIGS. 3a and 3b. Aperture is typical of those apertures formed at the top of the vertically disposed metal sheet and which are of the generally desired cross-sectional configuration. The cross section of the aperture is uniform throughout its circumference and more importantly lip portion 41 is of a constant width. Unfortunately, not all of the apertures produced by the chemical milling process are of this uniform cross-sectional configuration. For example as shown in FIG. 3b, aperture 45 has a non-uniform cross section. lt has a pronounced undercutting in por-. tions 46 and 47 and also lip portion 48 is not of a constant width, being thinner at the top of the aperture than at the bottom. Aperture 45 is illustrative of those produced at the bottom of the vertically disposed metal sheet. The differing cross sections of apertures 45 and is believed to result from aperture experiencing a greater degree of etchant run off than aperture 40. Note that the apertures formed at the bottom of the worksheet receive all of the etchant run off from those tures near the bottom as illustrated by aperture 45.

Why this actually occurs is unknown.

Even though these non-uniformities in cross- .sectional area occur, the aperture areas are still of proper size and in the proper size relationship from aperture to aperture.

Unfortunately, aperture 45 will enlarge at a faster 7 rate during re-etch than aperture 40. This is because of the narrow upper lip portion 48' of aperture 45 which will etch away faster than either the lower or upper lip portions of aperture 40. If all of the apertures of the shadow mask were re-etched under the same conditions, the apertures at the bottom of the mask, with the general cross-sectional configuration of aperture 45, will be too large in area and thus not be of the proper "size or in the proper size relationship from aperture to aperture. For convenience, the apertures, represented by aperture 45 which will enlarge at a faster rate upon re-etch will be defined as as apertures comprising a first set and those apertures represented by aperture 40 will be defined as apertures comprising a second set.

The inventive method re-etches the shadow mask in such a way that all the apertures achieve a final proper size in a final predetermined size relationship from aperture to aperture.

An apparatus embodying the present invention and which may be used in practicing the inventive method is shown in FIG. 4. The shadow mask 50 is supported by support means 51 in a substantially horizontal position with the convex side of the aperture mask facing downwardly. A plurality of spray nozzles 52-58 are arranged in an array and are located beneath shadow mask '50. The spray nozzles project an etchant, suitable for attacking the material of shadow mask 50 vertically upward to apply the etchant to the shadow mask in a substantially normal direction to the mask. The etchant is supplied to the spray nozzles through supply pipe which is connected to an etchant supply tank (not shown). The array of spray nozzles may be rotated by a rotating means (not shown) as represented by arrow 61 to compensate for any non-uniformity in the spray projected from the spray nozzles.

As explained earlier, the apertures comprising the first set, namely those enla'rgeable at a faster rate than those of the second set and having the general crosssectional configuration of aperture 45 of FIG. 3b, are

6 located generally at the bottom of the metal worksheet of FIG. 1. These apertures are represented as being located in area of shadow mask 50 of FIG. 4. As explained earlier, the difference in the enlargement rate of the two sets of apertures is compensated for. This is achieved by applying more etchant to the apertures of the second set. This may be accomplished by providing 5 shield 71 to inhibit the etchant from attacking those apertures comprising the first set. The linear array of spray nozzles is rotated and the etchant from these nozzles is applied to the mask for a sufficient amount of time to allow the second set of apertures to become larger in size than the apertures of the first set. Then the etchant from spray nozzles 52-58 isapplied to all of the apertures by removing shield 71 with removing means (not shown), or by placing the shadow mask into another etchant apparatus not having a shield. 5 To avoid having to remove shield 71 or placing the shadow mask into another etchant apparatus, the amount of etchant applied to the first and second set of apertures may optionally be controlled by perforating shield 71. Perforations 72 of shield 71 allow less etchant to reach the first set of apertures in region 70 than reaches the non-shielded portions of the shadow mask comprising the apertures of the second set. The dimension and size of the perforations may be adjusted so that the proper amount of etchant is applied to each set of apertures.

FIG. 5 shows a graph representing the typical difference in the rate of enlargement of the apertures as represented in FIGS. 3a and 3b. Plot represents a typical aperture-size-etch time relationship for an aperture having the general configuration of an aperture such as aperture 40 ofFlG. 3a. Plot 81 is a typical aperture size-etch time relationship for an aperture such as aperture 45 of non-uniform cross section and can be seen to have an etching rate greater than that of the aperture 40 represented in FIG. 3a. A graph such as the one represented in FIG. 5 can be arrived at by empirical analysis for deriving the particular time parameters to be used for the inventive process.

The present invention provides an apparatus and a method of producing a color selection electrode having a multiplicity of apertures of the proper size and the proper size relationship from aperture to aperture notwithstanding the differences in the cross-sectional configuration of the apertures after an initial chemical milling process.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

l claim:

1. In the manufacture of a color tube including a faceplate having an image screen and a shadow mask of etchable material provided with a multiplicity of apertures arranged in a predetermined pattern, certain ones of said apertures comprising a first set being susceptible to enlargement at a faster rate upon subsequent etching than certain other ones of said apertures comprising a second set, the method comprising the steps of:

printing a latent image of said pattern of apertures on said faceplate with said shadow mask; and subsequently enlarging the apertures of said shadow mask by differentially applying etchant thereto such that more etchant is applied to said second set than to said first set of apertures whereby said apertures are enlarged without disrupting said pattern.

2. A method in accordance with claim 1 where said etchant is differentially applied to said mask such that more of said etchant is applied to said second set of apertures than to said first set of apertures by first applying said etchant to said second set of apertures to thereby render said second set of apertures larger than said first set, and thereafter applying said etchant to all of said apertures.

3. A method in accordance with claim 2 where said etchant is first applied to said second set of apertures by spraying said etchant onto said mask while shielding said first set of apertures from said etchant spray.

4. A method in accordance with claim 1 where more of said etchant is applied to said second set of apertures than to said first set by spraying said etchant onto said mask while partially shielding said first set of apertures.

a plurality of spray nozzles. 

2. A method in accordance with claim 1 where said etchant is differentially applied to said mask such that more of said etchant is applied to said second set of apertures than to said first set of apertures by first applying said etchant to said second set of apertures to thereby render said second set of apertures larger than said first set, and thereafter applying said etchant to all of said apertures.
 3. A method in accordance with claim 2 where said etchant is first applied to said second set of apertures by spraying said etchant onto said mask while shielding said first set of apertures from said etchant spray.
 4. A method in accordance with claim 1 where more of said etchant is applied to said second set of apertures than to said first set by spraying said etchant onto said mask while partially shielding said first set of apertures.
 5. A method in accordance with claim 1 where said etchant is applied to said mask by spraying said etchant onto said mask in a substantially normal direction to said mask.
 6. A method in accordance with claim 5 where said etchant is sprayed onto said mask by a rotating array of a plurality of spray nozzles. 